1. Field of the Invention
The present invention relates to an image processing technique.
2. Description of the Related Art
Conventionally, techniques for transforming images and techniques for performing filter processing for images are independently known. As an example of the image transformation techniques, techniques (to be collectively referred to as distortion correction processing hereinafter) called, for example, distortion correction processing, keystone correction processing, or geometric correction processing are available. A distortion correction technique is employed in, for example, a projector. FIGS. 1A to 1D show how a projector 100 displays images on a screen 101. When the angle that the screen 101 makes with the optical axis of a projection optical system of the projector 100 is 90°, a projected image 102 is displayed on the screen 101 in a normal shape (for example, a rectangle). On the other hand, if this angle is not 90°, a projected image 103 is displayed on the screen 101 in a distorted shape (for example, a trapezoid). To correct this distortion, distortion correction processing is performed. FIG. 2 illustrates an example of the distortion correction processing. An input image 200 is converted into a distortion-corrected image 201 by the distortion correction processing. Note that the distortion-corrected image 201 includes a trapezoidal effective area 205 and a blanking area 204 other than the effective area 205. When the distortion-corrected image 201 is projected onto the screen 101, the effective area 205 is displayed on the screen 101 in a rectangle (image 203).
Various techniques of performing filter processing for images are known as well. As an example of the filter processing, visibility improvement processing which uses spatial filter processing is available. This visibility improvement processing includes improving the frame rate when a moving image is displayed (see, for example, an image 202 shown in FIG. 2). As an example of such processing, a technique described in Japanese Patent Laid-Open No. 2009-44460 is available.
The inventor of the present invention examined the use of distortion correction processing and filter processing in combination, and found that the following problems are posed in that case. The first problem will be described first with reference to FIG. 3A. Referring to FIG. 3A, a projected image 304 projected on the screen is obtained by performing filter processing for a distortion-corrected image 303, and projecting the processed image onto the screen. An area 301 in the distortion-corrected image 303 corresponds to an area 305 in the projected image 304. Also, an area 302 in the distortion-corrected image 303 corresponds to an area 306 in the projected image 304. Note that the areas 301 and 302 have the same size, while the areas 305 and 306 have different sizes. That is, when filter processing is performed for the distortion-corrected image 303 using the area 301 as a reference area, the projected image 304 appears to the observer as if it were obtained by performing filter processing for the distortion-corrected image 303 using the area 305 as a reference area. Similarly, when filter processing is performed for the distortion-corrected image 303 using the area 302 as a reference area, the projected image 304 appears to the observer as if it were obtained by performing filter processing using the area 306 as a reference area. In this manner, the projected image 304 appears to the observer as if it were obtained by nonuniform filter processing, and the observer may recognize this appearance as degradation in image quality. When, for example, smoothing is performed by filter processing, the image sharpness lowers more considerably in the area 305 than in the area 306.
The second problem will be described next with reference to FIG. 3B. When filter processing is performed for the distortion-corrected image 303 in reference to an area 300 at the boundary between a blanking area 307 and an effective area 308, the sharpness in the vicinity of this boundary may decrease. The pixels in the blanking area 307 normally have, for example, predetermined pixel values (R, G, B)=(0, 0, 0) (black). The central pixel of an area 310 falls within the blanking area 307, and its pixel values are obtained by filter processing in reference to the area 310. However, because the area 310 partially includes the effective area 308, the pixel values obtained by filter processing are different from the above-mentioned predetermined pixel values.
On the other hand, the central pixel of an area 309 falls within the effective area 308, and its pixel values are obtained by filter processing in reference to the area 309. However, because the area 309 includes the blanking area 307, the pixel values obtained by filter processing are influenced by those in the blanking area 307. That is, as described above, if the pixel in the blanking area 307 is black, the pixel values in the blanking area 307 are relatively large and those in the effective area 308 are relatively small at the boundary between the blanking area 307 and the effective area 308.